This invention relates to apparatus and method for reducing DC offsets in a direct conversion receiver.
For purposes of this disclosure it will be understood that xe2x80x9cdirect conversionxe2x80x9d receivers are communications receivers which receive radio frequency (RF) signals modulated with voice, data, etc. and convert the RF directly to a near DC Intermediate Frequency (IF) or spectrum of frequencies. Generally, the RF signals are received by a standard front end and coupled to a mixer where they are mixed with a local oscillator (LO) output and applied to one or more DC coupled amplifier chains. In DC coupled amplifier chains used in direct conversion receivers, DC offsets from the conversion mixer and components in the amplifier chain may, when amplified by the amplifier chain, saturate the output stages and suppress the processing of the desired base-band signal. In-line high pass filters (e.g., large AC coupling capacitors) introduce long settling times. The long settling times are especially of concern when the receiver is recovering from initial start-up transients, transients introduced by circuit configuration changes, or responding to DC component changes from the conversion mixer due to LO level and signal level variations across the different frequency bands.
In a software configurable radio receiver, the DC coupled amplifier chain or chains may be called upon to optimize operation with different signal-coding schemes, different RF input levels, and/or different signal bandwidths. In addition, the receiver front end may be reconfigured for different RF operating frequencies. As the coding schemes vary and mixer operating frequencies vary, differing operating characteristics will be required in the DC coupled amplifier chain or chains. Adapting to these different environments produces variable and possibly unpredictable DC offsets. In this environment, the DC coupled amplifier chain or chains has even more stringent requirements for DC balance than ever required on a single purpose receiver. With extended performance environment, the receiver requires the capability to suppress the DC signal components quickly and then pass the undistorted IF or base band signal.
Accordingly it is highly desirable to provide apparatus and a method of overcoming this problem which is inexpensive and easy to use.